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The authors report on the design and fabrication of high-efficiency GaN-based light emitting diodes (LEDs) with vertical-injection geometry. Based on the analyses of LED test patterns fabricated with various n-electrode dimensions, a design rule for vertical LEDs is proposed. It is found that the suppression of the vertical current under n electrodes and the efficient injection of the spreading current across the n layers are essential to fabricate high-efficiency LEDs. Introduction of the current blocking layer along with well-designed branched n electrodes results in a large enhancement of power efficiency by a factor of 1.9, compared with that of reference LEDs.

PACS 85.60.Bt, 85.60.Jb The external efficiency of normal LEDs is reduced by total internal reflection, which typically causes 50 to 80 percent of the emitted photons to be trapped in guided modes. We have increased the external quantum efficiency by using the corrugated interface substrate (CIS) texturing on the sapphire substrates to scatter light out of the nitride film. In order to maximize the outcoupling efficiency of the corrugation, we stimulated the outcoupling efficiency by varying the diameter, the period and the depth of the corrugations. With the use of the optimized corrugation pattern, the external quantum efficiency of 34% was achieved experimentally, without detriment to the forward and reverse electrical characteristics of LEDs.1 Introduction White LEDs are in high demand for the solid-state lighting technology, and they are expected to replace conventional incandescent and fluorescent lamps. There have been extensive investigations on the high efficiency UV LEDs as an excitation source [1,2]. Considering the extraction of spontaneous emission in LEDs, the photons emitted in the active region are mainly coupled into the high-index (GaN/sapphire) guided mode. Moreover, as the reflectivity of p-electrode is not unity, some portion of the propagating light is absorbed by the electrode on each reflection. Several methods have been suggested for minimizing the total internal reflection such as a random texturing on the top surface of LEDs or ordered micro-scale corrugation between GaN and sapphire interface [1,3,4]. However, the effects of corrugation size and shape have not been studied yet, even though it is a very challenging point.In this study, we have increased the external efficiency by using the corrugated interface substrate (CIS) to scatter light out of the nitride film. The corrugations are made by photolithography, followed by inductively coupled plasma reactive ion etching (ICP-RIE) to pattern grooves in sapphire. In order to maximize the outcoupling efficiency of the corrugation, we have analyzed the effects of shape and density of the patterns. Simulation was adopted for this purpose, which could easily vary the diameter, the interval and the depth. With the use of the optimized corrugation pattern, increase in the extraction efficiency as high as 70% was achieved experimentally, without detriment to the crucial electrical properties of LED such as V op and V r .

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